Thinking, braking and stopping distances, the factors that affect them, and how vehicle safety features reduce injury.

What this dot point is asking

WJEC Double Award Unit 6 wants you to describe thinking, braking and stopping distances, the factors that affect them, and how vehicle safety features reduce injury.

Thinking, braking and stopping distance

Factors affecting the distances

Why speed has a big effect

The braking distance depends on the car's kinetic energy ($E_k = \tfrac{1}{2}mv^2$), which depends on the square of the speed. So doubling the speed quadruples the braking distance, because there is four times as much kinetic energy for the brakes to remove. This is why small increases in speed cause large increases in stopping distance, and why speed limits matter so much for safety.

How braking transfers energy

When a car brakes, the brakes do work against friction to slow it down. The car's kinetic energy is transferred to heat in the brakes, which is why brakes get hot. The greater the kinetic energy (the faster or heavier the car), the more work the brakes must do and the longer the braking distance. Worn brakes or tyres reduce the friction force, so it takes longer to stop. This links stopping distance to work and energy.

Vehicle safety features

Safety features reduce injury by increasing the time over which a person (or the car) slows down. A longer time means a smaller force on the person (a gentler change in motion):

• Crumple zones crumple on impact, so the car takes longer to stop.
• Seatbelts stretch slightly and stop the person hitting the inside of the car, spreading the force over more time and a larger area.
• Airbags cushion the impact, increasing the time and spreading the force.

Reaction time and reaction tests

The thinking distance depends on the driver's reaction time - the time between seeing a hazard and acting. Reaction time can be measured with a simple practical: a ruler is dropped between someone's fingers and the distance it falls before they catch it shows how quick their reaction is (a longer drop means a slower reaction). Reaction time is increased by tiredness, alcohol, drugs and distractions, all of which increase the thinking distance and so the overall stopping distance. Linking a slower reaction time to a longer thinking distance is a common exam point.

Why stopping distance matters for road safety

Understanding stopping distances explains many road-safety rules. Speed limits keep braking distances manageable, especially near schools and in towns. Drivers are told to leave a larger gap in wet or icy conditions because the braking distance is longer, and to slow down when tired because the thinking distance is longer. The big increase in braking distance with speed (because it depends on speed squared) is the scientific reason behind "speed kills" campaigns. Being able to apply stopping-distance ideas to real road-safety advice is often rewarded in exams.

Try this

Q1. What two distances make up the stopping distance? [1 mark]

• Cue. Thinking distance and braking distance.

Q2. Give one factor that increases thinking distance. [1 mark]

• Cue. Any one of: tiredness, alcohol, drugs, or distraction.